This is currently (Sep-2023) surprisingly difficult compared to Google Sites or Confluence...

There is no way of adding a ToC based on all existing heading, you need to add each one manually. Here's the quickest way to achieve this...

First you need to know the names of the anchor links for each heading you want to link to. These normally consist of all the words in the heading in lowercase with all spaces replaced by "-". You can see the link names by hovering over the link icon next to each heading on your published site.

Add a text block. Press Ctrl-K to add a hyperlink. Add the anchor link name in the Address field. e.g.

#my-anchor-link-name

Note that the Search functionality in this dialog only finds page links not anchor links.